It is well known that vinyl aromatic monomer compounds such as styrene or .alpha.-methylstyrene, have a strong tendency to polymerize when subjected to elevated temperatures. Manufacturing processes for such monomers typically include distillations at elevated temperatures.
To prevent premature polymerization of vinyl aromatic monomers during the distillation purification process, various compounds have been disclosed as polymerization inhibitors. These include elemental sulfur and many classes of organic chemicals. These materials have met with varying degrees of success in industrial use. Included among these organic materials are nitrated phenol derivatives, C- and N-nitroso compounds and nitroxyl derivatives. A particularly effective class of such inhibitors are hindered amine nitroxyls. This class of stable free radicals is characterized by having a NO* group, where the * asterisk denotes an unpaired electron, and where the nitrogen atom is further flanked by two carbon atoms, to neither of which hydrogen atoms are attached. Often these flanking carbon atoms are further connected by various bridging groups to form cyclic structures, such as, for example, six-membered piperidines, piperazines, five membered pyrollidines and the like. Collectively these stable free nitroxyl radicals (also called nitroxides) are referred to as "hindered amine" nitroxyl free radicals. Compounds of this class are not only valuable inhibitors of free radical-initiated polymerization in unsaturated monomers, but also have found uses as spin labels in biochemical applications.
Soviet Patent No. 1,027,150 discloses the stabilization of styrene by using nitroxyl radical. Soviet Patent No. 1,139,722 discloses the use of a bis-nitroxyl radical as the thermal polymerization inhibitor for styrene. Japanese Hei 1-165534 discloses the use of 1-piperidyloxy derivatives as polymerization inhibitors for styrene. Soviet Patent No. 1,558,888 discloses the polymerization inhibition of styrene by a nitroxyl radical. U.S. Pat. No. 3,733,326 discloses the polymerization inhibition of vinyl monomers by free radical precursors. The inhibition of styrene polymerization by some hindered amine nitroxyl derivatives is also discussed by Y. Miura, S. Masuda, and M. Kinoshita, Die Makromolekulare Chemie, 1972, 160, 243-249, M. D. Gold'rein, E. A. Ragikov, V. N. Kozhevnikov, A. D. Stepukhovich and A. V. Trubnikov, Vysokomol.soyed, 1975, A17 (8), 1671-1677 translated in Polymer Science (USSR), 1975, A17(8), 1919-1927 and by G. Moad, E. Rizzardo, and D. H. Solomon, Polymer Bulletin 1982, 6, 589-593. Inhibition of polymerization of vinyl aromatic compounds by a combination of a stable nitroxyl compound with aromatic nitro compounds is the subject of U.S. Pat. No. 5,254,760 (1993).